Question

I downloaded "Firefox Setup 19.0.exe" and tried to install Firefox 19 on Windows XP (serv.pack 3) but the setup program refuses to complete installation (It only decompress some files at folder "user\config\temp\7zS105.tmp" then delete all files and close without an error message).

I found "Firefox Setup 19.0b6.exe" at ftp.mozilla.org and it worked well. The installation setup was completed.

What was going on? Could the default setup be more "expressive" and tell about the error?

Sorry but it is not a failed downloaded file, it is some error at "Firefox Setup 19.0.exe". It is only extracting its files at some temporary folder at "temp" windows folder and then, for some unknown reason, stop installation and deletes all files and folders created.

It is possible that anti-virus software is corrupting downloaded files or is otherwise interfering with downloading the file an the installation.
Try to disable your anti-virus software or only the real-time scan temporarily to see if that makes things work if it happens again.